This invention relates to a fluoroelastomer composition containing a phosphonate accelerator for curing the fluoroelastomer.
Fluoroelastomers containing units of vinylidene fluoride and other fluorine-containing monomers, such as hexafluoropropylene with or without tetrafluoroethylene, that are cured with quaternary phosphonium accelerators which are alkyl- or aralkyl triarylphosphonium compounds, for example, benzyltriphenyl phosphonium chloride, in combination with a crosslinking agent, such as a Bisphenol, and a basic metal oxide or hydroxide, have been known for many years. The vulcanizates of such compositions possess excellent physical properties and resistance to environmental attack and are used to manufacture such articles as gaskets, O-rings, wire insulation, hoses, etc. Unfortunately, however, the curable fluoroelastomers containing quaternary phosphonium accelerators of the type described above cause mold fouling. Mold fouling is a process by which metal molds are contaminated by the curable fluoroelastomer stock that is to be molded or by its degradation products during the curing or vulcanization process. When the cured fluoroelastomer composition has been removed from the mold cavity, a black deposit which appears to be fluoroelastomer stock is observed adhering to the surface of the metal mold cavity. After as few as twenty-five molding cycles, the amount of fouling of the mold cavity may be sufficient to be weighed with a microbalance. Mold fouling during curing of the fluoroelastomers eventually requires the fabricator to shut down the mold line so that the mold can be cleaned; otherwise, the molded articles have a rough, uneven surface. Metal corrosion is believed to be a factor in mold fouling. It is believed that acids which are given off during conventional curing of vinylidene fluoride-containing fluoroelastomer compositions can cause micro-cracking, even in mold cavities made of austenitic stainless steel, and fouling soon follows. The present invention provides novel fluoroelastomer compositions containing phosphonate accelerators for curing the fluoroelastomers. The phosphonate accelerators substantially reduce mold fouling when the elastomers are cured in steel mold cavities. At the same time, the use of these accelerators in the fluoroelastomers does not compromise the important physical properties of the elastomers, such as compression set resistance, percent elongation at break, and tensile strength at break. Furthermore, the phosphonate accelerators used in the present invention provide cure rates which permit commercially feasible rates of production of fluoroelastomer parts.